Flexible stabilization device for dynamic stabilization of bones or vertebrae

ABSTRACT

A flexible stabilization device for dynamic stabilization of bones or vertebrae is provided comprising a rod construct including a rod made of an elastomeric material the rod having a first connection section, a second connection section and a third section therebetween, the first and second connection sections being connectable with a bone anchoring device, respectively, and a sleeve provided on at least a portion of the third section of the rod such that at least the first and second connection sections are exposed.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.13/425,153, filed Mar. 20, 2012, which is a continuation of U.S. patentapplication Ser. No. 11/642,566, filed Dec. 19, 2006, now U.S. Pat. No.8,157,843, which claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/753,620, filed Dec. 23, 2005, and claimspriority from European Patent Application EP05028283, filed Dec. 23,2005, the entire disclosures of which are incorporated herein byreference.

BACKGROUND

The present disclosure relates to a flexible stabilization device forthe dynamic stabilization of bones or vertebrae.

A flexible stabilization device for stabilizing adjacent vertebrae isknown from EP 0 669 109 B1. In this stabilization device monoaxial bonescrews placed in adjacent vertebrae are connected by an elastic strap.The strap is fastened to the bone screws in a pre-stressed manner. Asupport body which is resistant to compression surrounds the strapbetween the bone screws to transmit compressive forces. The supportbody, the heads of the bone screws and the elastic strap form a kind ofjoint allowing a limited motion of the vertebrae.

US 2003/0220643 A1 discloses a device for connecting adjacent vertebralbodies in which monoaxial pedicle screws are interconnected by a spring.The spring allows spinal flexion and a limited degree of lateral bendingand axial rotation while preventing spinal extension without the need ofa transverse member. A sleeve is placed over the spring. Impingementbetween the sleeve and the pedicle screws assists the spring inpreventing spinal extension. The length of the spring is predetermined.An adaptation in length by the surgeon is not possible.

WO 2004/105577 A2 discloses a spine stabilization system with one ormore flexible elements having an opening or slit in form of a helicalpattern. Adjustments of the system with regard to its flexiblecharacteristics are not possible during surgery.

A bone anchoring device comprising a monoaxial bone screw and a flexiblerod which is made of an elastic material is known from EP 1 364 622 A2.The elastic characteristics of the bone anchoring device are determinedby material and the shape of the rod which cannot be modified by thesurgeon. Furthermore, the use of monoaxial bone screws limits thepossibility of adjustment of the position of the shaft relative to therod.

Based on the above, there is a need for a flexible stabilization devicefor dynamic stabilization of bones or vertebrae which allowsmodification of the elastic characteristics of the device and at thesame time the adaptation of the length of the rod construct during thesurgical operation.

SUMMARY OF THE INVENTION

A flexible rod assembly including an inner rod and outer rod or sleevemade of an elastomeric material allows an adjustment of the elasticcharacteristics of the stabilization device to a large extent. By meansof selection of a rod and a sleeve with appropriate elastic propertieswhich can be different from each other an adaptation of the elasticproperties of the rod construct to the motion of a specific spinalsegment is possible. In particular, flexion and compression of the spinecan be controlled by means of the elastic properties of the inner rod,whereas extension of the spine can be controlled by selection of anappropriate sleeve. The separation of the damping with regards toflexion/compression and extension movements results in a harmonicbehaviour of the vertebral segments under motion control of theconstruct. As a consequence thereof loosening of the bone screws can beprevented. Additionally, adjustment of the length of the inner rod andof the sleeve is possible. Hence, a modular system is provided which isallows adaptation at the time of surgery. In combination with polyaxialscrews the possibilities of adjustment are further increased.

Further features and advantages of the disclosure will become apparentand will be best understood by reference to the following detaileddescription of embodiments taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a perspective exploded view of a rod construct accordingto an embodiment of the disclosure.

FIG. 1 b shows the rod construct of FIG. 1 a in an assembled state.

FIG. 2 shows an exploded view of a stabilization device comprising therod construct of FIG. 1 a.

FIG. 3 schematically shows in an exploded view the accommodation of therod of FIG. 1 a in the receiving part of a polyaxial bone screw.

FIG. 4 shows a sectional view of the assembled parts of FIG. 3.

FIG. 5 schematically shows the assembled stabilization device of FIG. 2applied to adjacent vertebrae of the spinal column.

FIG. 6 a schematically shows the stabilization device of FIG. 5 with thespinal column in flexion.

FIG. 6 b schematically shows the stabilization device of FIG. 5 with thespinal column in extension.

FIG. 7 schematically illustrates the directions of displacement of therod construct of FIG. 1 b.

FIG. 8 a schematically shows the rod construct in a state of flexionaccording to FIG. 6 a.

FIG. 8 b schematically shows the rod construct in a state of extensionaccording to FIG. 6 b.

FIGS. 9 a and 9 b show a further example of application of thestabilization device in a top view.

FIG. 10 shows modification of the rod construct in section.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIGS. 1 a and 1 b, the flexible stabilization deviceincludes a rod assembly including rod 20 made of an elastomeric materialand a sleeve 40 which is also made of an elastomer material. In theembodiment shown, the rod 20 has a cylindrical shape with a smoothsurface. Due to the elastomer material, the rod is partially or fullyflexible. For example, the rod 20 can be made of a biocompatible plasticmaterial such as a polymer on the basis of polyurethane, polysilicone orPEEK. A particularly suitable material is Polycarbonate Urethane. Thematerial of the rod includes well-defined elastic properties and the rodshows bending elasticity, compressive elasticity and tensile elasticity.

The elastomeric material of the sleeve 40 can also be a biocompatibleplastic material such as a polymer on the basis of polyurethane,polysilicone or PEEK which includes elastic properties which can beselected independently of the elastic properties of the rod 20. Also forthe sleeve 40, Polycarbonate Urethane is particularly suitable. Thesleeve 40 has a tube-like shape including a channel 41 the diameter ofwhich is slightly larger than the outer diameter of the rod 20 so thatthe rod 20 can be inserted into the channel 41 as shown in FIG. 1 b. Thelength of the sleeve 40 is selected to be smaller than the length of therod 20 such that a first connection section 20 a and a second connectionsection 20 b of the rod 20 protrude from the channel 41 in the assembledstate as shown in FIG. 1 b. A section 20 c between the first connectionsection 20 a and the second connection section 20 b of the rod 20 isaccommodated in the channel 41 of the sleeve 40. Preferably the lengthof the sleeve 40 corresponds approximately to the distance between thereceiving parts of the bone anchoring devices, or can be slightlylarger.

With reference to FIGS. 2 to 4 the connection of the rod 20 with thereceiving part 6 of a bone anchoring element 1 is explained. Althoughthe sleeve 40 is omitted in the illustration of FIG. 3, for the purposeof describing the connection of the rod 20 with the receiving part 6,the sleeve 40 is placed on the rod 20 before the latter is secured tothe respective receiving parts 6 of the bone anchoring elements 1.

The bone anchoring element 1 in this embodiment is a polyaxial bonescrew having a shank 2 with a bone thread, a tip 3 at one end and aspherical head 4 at the opposite end. A recess 5 for engagement with ascrewing-in tool is provided at the side of the head 4 which is oppositeto the shank. The receiving part 6 has a first end 7 and a second end 8opposite to the first end and a longitudinal axis 9 intersecting theplane of the first end and the second end. Coaxially with thelongitudinal axis 9 a bore 10 is provided which extends from the firstend 7 to a predetermined distance from the second end 8. At the secondend 8 an opening 11 is provided the diameter of which is smaller thanthe diameter of the bore 10. A spherical or otherwise tapering section12 is provided adjacent of the opening 11 which forms a seat for thespherical head 4.

The receiving part 6 further has a U-shaped recess 13 which starts atthe first end 7 and extends in the direction of the second end 8 to apredetermined distance from said second end 8. By means of the U-shapedrecess 13 two free legs 14, 15 are formed extending towards the firstend 7. Adjacent to the first end 7 the receiving part 6 comprises aninternal thread 16 on said legs 14, 15.

As can be seen in FIG. 3, a first pressure element 17 is provided whichhas a cylindrical construction with an outer diameter which is onlyslightly smaller than the inner diameter of the bore 10 to allow thefirst pressure element 17 to be introduced into the bore 10 of thereceiving part 6 and to be moved in the axial direction. On its lowerside facing towards the second end 8, the pressure element 17 includes aspherical recess 18 the radius of which corresponds to the radius of thespherical head 4 of the bone screw. On the opposite side, the firstpressure element 17 has a cylindrical recess 19 which extendstransversely to the longitudinal axis 9. The lateral diameter of thisrecess is selected such that the connection section 20 a or 20 b with acircular cross section, respectively, of the rod 20 which is to bereceived in the receiving part 6 can be inserted into the recess 19 andguided laterally therein. The depth of the cylindrical recess 19 isselected such that in an assembled state when the connection section 20a or 20 b of the rod 20 is inserted and pressed against the bottom ofthe U-shaped recess 13, the first pressure element 17 exerts a pressureon the head 5. Further the depth is preferably about half of thediameter of the connection section 20 a or 20 b of the rod 20. As can beseen in FIG. 3, the first pressure element 17 has a coaxial bore 21 forguiding a screwing-in tool therethrough.

As shown in FIGS. 3 and 4, the bone anchoring element 1 furthercomprises a second pressure element 23 with a first end 24 and a secondend 25. The width of the second pressure element 23 is such that thesecond pressure element 23 can be inserted into the U-shaped recess 13of the receiving part 6. On opposite sides of the second pressureelement 23 two cylindrical projections 26 are provided which fit intothe space limited by the internal thread 16 to slide along the internalthread 16 when the second pressure element 23 is inserted.

As can be seen in FIG. 2, the second pressure element 23 furtherincludes a cylindrical recess 27 extending from the second end 25 in thedirection towards the first end 24 the cylinder axis of which isperpendicular to that of the cylindrical projections 26. On the side ofthe second end 25, the cylindrical projections 26 include lower edges 26a. The diameter of the cylindrical recess 27 corresponds to the diameterof the connection section 20 a or 20 b of the rod 20 and its depth tohalf or less than half of the diameter of the connection section 20 a or20 b.

The bone anchoring element 1 further includes an inner screw 30 whichcan be screwed-in between the legs 14, 15. The internal thread 16 andthe cooperating thread of the inner screw 30 can have any known threadshape. Using a flat thread or a negative angle thread can preventsplaying of the legs 14, 15.

The receiving part 6 and the first pressure element 17 can havecorresponding crimp bores 32, 33 on opposite sides by means of which thescrew 1, the receiving part 6 and the first pressure element 17 can beloosely pre-assembled. As shown in FIGS. 3 and 4 the first pressureelement 17 and the second pressure element 23 can have projections 22,28, respectively, which can contribute to the fixation of the elasticrod 20.

The other parts of the flexible stabilization device except the rod 20and the sleeve 40 can be made of the commonly used biocompatiblematerials, such as stainless steel or titanium or any other materialsuitable for a bone screw.

In use, at least two bone anchoring devices are anchored into the bone.Next, the rod 20 and the sleeve 40 are selected and combined to achievethe desired elastic properties of the flexible stabilization device. If,for example, more than two vertebrae are to be connected, differentsleeves 40 having different elastic properties can be selected andprovided between different vertebrae. In this way, the elasticproperties of the stabilization device can be adapted at the time ofsurgery. Preferably, the sleeve 40 is selected to have a lengthcorresponding to the distance of the two receiving parts when thepedicle screws are screwed into adjacent vertebrae. Since the rod 20 andthe sleeve 40 are made of elastomeric material, shortening duringsurgery is possible. Then, rod 20 with the sleeve 40 or, if more thatone motion segment shall be stabilized via a single rod 20, with aplurality of sleeves 40 is inserted into the receiving parts 6 of thebone anchoring elements. Preferably, in the balanced position of the twoadjacent vertebrae, the sleeve 40 is in contact with the receiving parts6.

Thereafter, the second pressure element 23 is inserted in the receivingpart 6 and the inner screw 30 is screwed-in between the legs 14, 15.After adjusting the angular position of the bone screw, the inner screw30 is tightened. By the pressure exerted by the inner screw 30 onto thesecond pressure element 23, the rod 20 is clamped between the first andthe second pressure element 17, 23 and simultaneously the head 4 of thebone screw is locked in its angular position.

Next, with reference to FIGS. 5 to 8 b the elastic properties of theflexible stabilization device are described. In FIG. 5 the assembledstabilization device is shown with the rod 20 and the sleeve 40 arrangedto connect two polyaxial pedicle screws which are placed in adjacentvertebrae W forming a motion segment. The positions of the shanks of thepedicle screws are indicated by dash-dotted lines. As can be seen inFIG. 5, the receiving parts 6 of the bone anchoring elements 1 have adistance x in the balanced position in which the rod 20 and the sleeve40 are in an unstressed state.

FIG. 6 a shows the stabilization device when flexion takes place. Duringflexion, tensile stress is applied to the rod 20 resulting in anelongation of the rod 20. The distance between the bone anchoringelements is increased to x+ΔX₁. The increase ΔX₁ in the distance islimited by the restoring force produced by the rod 20 due to its elasticproperties. The increase in the distance can be, for example, in therange of approximately 1.5 mm. Hence, flexion/compression is controlledmainly by the inner rod 20.

FIG. 6 b shows the stabilization device when extension takes place.During extension, a compressive force is applied to the rod 20 and thesleeve 40 by the receiving parts 6 of the bone anchoring elements 1. Theelasticity of the rod 20 and the sleeve 40 allows the distance betweenthe receiving parts 6 to decrease to a distance x−Δx₂. Due to theelastic properties of the rod 20 and the sleeve 40, a restoring forceacts on the receiving parts 6 which limits the decrease of the distance.The distance can, for example, decrease by approximately 0.5 mm. Hence,extension is controlled by the compressibility of the inner rod 20 andis limited by the sleeve.

In an alternative manner of application, the sleeve 40 can bepre-compressed in the balanced state and/or the rod 20 can bepre-stressed in the balanced state.

FIG. 7 illustrates the possible deformations which the rod 20 and thesleeve 40 can undergo. FIGS. 8 a and 8 b show the deformation of the rod20 and the sleeve 40 in flexion (FIG. 8 a) and in extension (FIG. 8 b).

FIGS. 9 a and 9 b show an example of application of the stabilizationdevice. FIG. 9 a shows two adjacent vertebrae V, V′ which aremedio-laterally inclined with respect to each other in the case of thepresence of scoliosis. To dynamically stabilize and correct such amotion, segment rods 200, 200′ with different sleeves 400, 400′ can beused on the left side and on the right side. The sleeve 400 used on theleft side rod 200 has a length which is greater than the length ofsleeve 400′ used on the right side rod 200′. In this manner, it ispossible to eliminate the inclination of two vertebrae on the left side.In addition, the outer diameter of the sleeve 400 can be different fromthat of the sleeve 400′ in order to have a different motion control withrespect to the left side and the right side.

Further modifications of the above described embodiments are possible.In the embodiment described before, the sleeve 40 has the shape of ahollow cylinder; however, different shapes of the sleeve are possible.For example, a barrel-shape is possible. The length of the sleeve candiffer from the embodiment shown. The rod 20 may also have arectangular, square, oval or triangular cross-section or any otherappropriate shape of the cross-section. In this case, the shape of thesleeve 40 is appropriately adapted. In particular, it is possible toform the rod 20 and/or the sleeve 40 with the shape varying in thelongitudinal direction. The rod 20 and the sleeve 40 can be formed to behighly flexible or hardly flexible.

The surface of the rod 20 and/or the sleeve 40 can be textured orstructured. FIG. 10 shows an example of an inner rod 201 having acorrugated surface, with corrugations 300 provided in thecircumferential direction. The inner wall of the sleeve 401 hascorresponding corrugations cooperating with that of the rod. Thisprevents or reduces a displacement of the sleeve relative to the rod.

In the example of the bone anchoring element described above, theconnection of the shanks 2 of the bone anchoring elements 1 to therespective receiving parts 6 is polyaxial. However, it is also possibleto provide a monoaxial connection.

For the inner screw 30, all known modifications can be used. Thisincludes also the use of an outer ring or nut.

In the embodiments described, the bone anchoring element 1 is introducedfrom the top into the receiving part 6. However, the bone anchoringelement 1 can also be introduced from the bottom of the receiving part 6if the receiving part 6 is constructed to allow this.

The head of the bone anchoring element and the shaft can be constructedas separate parts which can be connected.

The present disclosure is not limited to screws as bone anchoringelements but can be realized with bone hooks or any other bone anchoringelement.

What is claimed is:
 1. A flexible stabilization device for dynamicstabilization of bones or vertebrae comprising: a rod assembly includinga rod made of an elastomeric material and structured such that itslength is adjustable, the rod configured to transmit compressive forcesin the axial direction, the rod having a first connection section, asecond connection section and a third section therebetween, the firstand second connection sections being connectable with a bone anchoringdevice, respectively, and a sleeve provided on at least a portion of thethird section of the rod, the length of the sleeve being smaller than alength of the rod, such that at least the first and second connectionsections are exposed.
 2. A flexible stabilization device according toclaim 1, wherein the sleeve is made of an elastomeric material.
 3. Aflexible stabilization device according to claim 2, wherein the elasticproperties of the rod and the sleeve are different.
 4. A flexiblestabilization device according to claim 2, wherein the elastomericmaterial of any one of the rod and the sleeve is a biocompatible plasticmaterial such as polyurethane or polysilicone.
 5. A flexiblestabilization device according to claim 1, wherein the inner wall of thesleeve is in contact with the surface of the rod.
 6. A flexiblestabilization device according to claim 5, wherein the inner wall of thesleeve has a structure which engages with a structure on the surface ofthe rod.
 7. A flexible stabilization device according to claim 1,further comprising at least a first and a second bone anchoring deviceconnected with the rod assembly.
 8. A flexible stabilization deviceaccording to claim 7, wherein at least one of the bone anchoring devicesis constructed so as to allow a polyaxial connection between a shank ofthe bone anchoring device and the rod.
 9. A flexible stabilizationdevice according to claim 1, wherein the rod comprises a plurality ofconnection sections and a plurality of sleeves therebetween.
 10. A rodassembly for a flexible stabilization device, the rod assemblycomprising: a rod made of an elastomeric material, the rod having afirst connection section, a second connection section and a thirdsection therebetween; and a sleeve provided on at least a portion of thethird section of the rod such that at least the first and secondconnection sections are exposed; wherein an inner wall of the sleeve hasa structure which engages with an inner structure on a surface of therod.
 11. A rod assembly according to claim 10, wherein the sleeve ismade of an elastomeric material.
 12. A rod assembly according to claim11, wherein the elastic properties of the rod and the sleeve aredifferent.
 13. A rod assembly according to claim 11, wherein theelastomeric material of any one of the bar and the sleeve is abiocompatible plastic material such as polyurethane or polysilicone. 14.A rod assembly according to claim 10, wherein the rod comprises aplurality of connection sections and a plurality of sleevestherebetween.
 15. A flexible stabilization device for dynamicstabilization of bones or vertebrae comprising: a rod assemblycomprising: a rod made of an elastomeric material and having a firstconnection section, a second connection section and a third sectiontherebetween; and a sleeve provided on at least a portion of the thirdsection of the rod, the length of the sleeve being smaller than a lengthof the rod, such that at least the first and second connection sectionsare exposed; a bone anchoring device connectable with any one of thefirst connection section and the second connection section of the rod,the bone anchoring device comprising: a shank portion configured to beanchored in a bone or in a vertebra; a head portion having a recessincluding an opening, the recess configured to receive a portion of anyone of the first connection section and the second connection section ofthe rod; and a securing element configured to secure the portion in therecess by pressure exerted on the portion.
 16. A flexible stabilizationdevice according to claim 15, wherein the sleeve is made of anelastomeric material.
 17. A flexible stabilization device according toclaim 16, wherein the elastic properties of the rod and the sleeve aredifferent.
 18. A flexible stabilization device according to claim 16,wherein the elastomeric material of any one of the bar and the sleeve isa biocompatible plastic material such as polyurethane or polysilicone.19. A flexible stabilization device according to claim 15, wherein theinner wall of the sleeve is in contact with the surface of the rod. 20.A flexible stabilization device according to claim 19, wherein the innerwall of the sleeve has a structure which engages with a structure on thesurface of the rod.
 21. A flexible stabilization device according toclaim 15, wherein at least on of the first bone anchoring device and thesecond bone anchoring device is constructed so as to allow a polyaxialconnection between the shank thereof and the rod.
 22. A flexiblestabilization device according to claim 15, wherein the rod comprises aplurality of connection sections and a plurality of sleevestherebetween.